An inverter driving type permanent-magnet synchronous motor, for example, includes a stator, which generates a rotational magnetic field when applied with alternating current by means of a switching element, and a rotor, which accommodates a permanent magnet therein and which is arranged to be rotatable relative to the stator.
The stator includes a yoke portion and plural tooth portions. The yoke portion is adapted to accommodate the rotor therein. Each of the tooth portions is provided at an inner circumferential surface of the yoke portion to radially protrude therefrom towards an axial center of the stator. Such stator includes wide portions, at which the tooth portions are formed, and narrow portions, at which the tooth portions are not formed. A width of each of the wide portions is arranged to be wider than that of each of the narrow portions because the tooth portions are formed at the wide portions. A stator coil is formed by directly winding wires on the tooth portions.
Due to such motor, an iron core may be provided at the stator and the rotor. The iron core is formed by laminating plural electromagnetic steel plates in an axial direction of the motor. The stator formed in such a manner is assembled to a case serving as an outer frame of the stator, for example, by shrink fitting. Further, the rotor is assembled to a rotational shaft, for example, by the shrink fitting.
For example, when the stator including the iron core formed by plural thin electromagnetic steel plates is assembled to the case by shrink fitting, a deformation such as buckling may be easily generated at outer electromagnetic steel plates, which are located at outward sides in a laminating direction. In order to prevent the generation of such deformation, for example, end plates made of non-magnetic material may be provided at both end portions of the laminated electromagnetic steel plates. However, in such a condition, a size of the motor is enlarged by the dimension of the end plates.
JP2005-151648A (hereinafter, referred to as reference 1) discloses a motor including an iron core. The iron core includes a first portion formed by laminating first electromagnetic steel plates and a second portion(s) formed by laminating second electromagnetic steel plates. The first portion and the second portion of the iron core are arranged to be adjacent to each other. Further, a thickness of each of the second electromagnetic steel plates is larger than that of each of the first electromagnetic steel plates.
So configured, a deformation of the first electromagnetic steel plates, which may be generated when the stator is assembled to the case, is prevented by the second portion(s). Further, the structure employing the end plates made of the non-magnetic material is not required, so that an increase of a size of the motor can be restrained.
In general, the iron loss of the motor is indicated with the sum of the hysteresis loss and the eddy-current loss. The degree of the eddy-current loss is proportional to a squared value of plate thickness. Therefore, when the thickness of the electromagnetic steel plates is increased, the ratio of the eddy-current loss is also increased.
The motor according to the reference 1 employs the electromagnetic steel plates (the second electromagnetic steel plates) of which thickness is increased. Therefore, the iron loss of the motor is also increased. Further, the motor according to the reference 1 is structured with the two types of electromagnetic steel plates, i.e., the first and second electromagnetic steel plates of which thickness are different from each other, thereby increasing material cost. Still further, the first and second electromagnetic steel plates are integrated after separately laminating the first electromagnetic steel plates and the second electromagnetic steel plates. Therefore, an assembling efficiency may be deteriorated.
Further, according to the motor of the reference 1, the stator is surrounded by the case, and a communicating passage may be required to be provided at the case. The communicating passage is formed by recessing an inner circumferential surface of the case and extends in the axial direction of the stator. The communicating passage is employed for a signal line which is to be inserted thereinto, or employed as an oil passage, for example. In a condition where the stator is assembled to the case which does not include any communicating passage by the shrink fitting or press fitting, a shrinking force acts evenly on an entire outer circumferential surface of the stator in a radial direction from the inner circumferential surface of the case. However, when applying the above described case including the communicating passage, there also exists a portion where the communicating passage is not formed. Accordingly, in such a condition, the shrinking force does not act evenly along whole the outer circumferential surface of the stator. The stator includes the wide portions, at which the tooth portions are formed, and the narrow portions, at which the tooth portions are not formed and of which rigidity is low. Therefore, deformation such as buckling may be easily generated at such narrow portions when assembling the stator into the case.
A need thus exists for a motor, which is not susceptible to the drawback mentioned above.